1. Technical Field
The present invention relates to a fisheye lens unit.
2. Related Art
Fisheye lens units including a plurality of lenses that constitute a fisheye lens are incorporated on still cameras and monitoring cameras, and are widely used to photograph wide area spaces.
Here, the fisheye lens represents a lens having a field angle of 160 degrees or greater, and normally having a field angle of 180 degrees or greater. The fisheye lens differs from a wide angle lens in that distortion is generated intentionally.
A specified projection method is used for the fisheye lens. For example, the following kinds of projection methods are well known.                (1) Orthogonal projection method - - - y=f·sin θ        (2) Equisolid angle projection method - - - y=2·f·sin(θ/2)        (3) Equidistant projection method - - - y=f·θ        (4) Stereographic projection method - - - y=2·f·tan(θ/2)        
Here, f is the focal length of the fisheye lens, θ is the incident angle of light incident to the fisheye lens, and y is the height of the image formed on the image surface (image height).
FIG. 1 shows a relationship of an incident angle and an image height according to the projection methods. The horizontal axis indicates the incident angle θ (radian), and the vertical axis indicates the image height y (mm). Curves C1 to C4 respectively represent the projection methods (1) through (4). However, in FIG. 1, for convenience, a focal length f is set to 1 mm. Note that, curves CA and CZ (described later) are also depicted in FIG. 1.
As shown in the drawing, near the incident angle of 90 degrees, a variation (Δy/Δθ) represented by an increment Δy of the image height y in relation to an increment Δθ of the incident angle θ becomes larger as it goes from the projection method (1) (curve C1) to the projection method (4) (curve C4). Namely, as it goes from the projection method (1) (curve C1) to the projection method (4) (curve C4), the ratio of the amount of information for the peripheral part inside the image increases. Therefore, if a large amount of information for peripheral part is required, it is preferable to use the projection methods (3) and (4) described above.
However, when using the projection methods (3) and (4), it was conventionally necessary to use about ten lenses for constituting the fisheye lens unit. Namely, when using the projection methods (3) and (4), it was relatively difficult to make the number of lenses smaller for constituting the fisheye lens unit.